1. Field of the Invention
This invention relates to a method and apparatus for indicating the presence of ice on a surface.
2. Description of the Related Art
Ice on the surface of an aircraft wing reduces aerodynamic performance and may, therefore, constitute a safety hazard.
One system for detecting ice on a wing surface is described in U.S. Pat. No. 5,180,122 to Christian issued Jan. 19, 1993. The system disclosed in Christian has a camera which receives the light reflected from an aircraft wing and passes the full spectrum to a video imager which outputs a blue video signal having an intensity dependent upon the intensity of the full spectrum. Another camera has a filter passing a narrow band of light centered about 1.8 microns. This filtered light is used to generate a green video signal. A third camera passes a narrow band of light centered about 1.6 microns and a red video signal is constructed from this filtered light. Christian considers that ice has a peak in its reflectance spectra at 1.8 microns and dips in its spectrum at 1.25, 1.6 and 2.0 microns. Christian suggests certain other substances on the wing do not have a reflectance spectra dip at 1.6 microns and, consequently, areas of the wing surface covered ice have a distinguishable colour on a video monitor driven with the composite video signal.
This invention seeks to provide an improved electro-optic ice detection apparatus.